simultaneously, and reduces turnaround time for multiple samples. Without multiplexing, information can be missed from run-to-run, affecting identification and quantification, as peptides selected for fragmentation on one LC-MS/MS run may not be present or of suitable quantity in subsequent sample runs. The current available isobaric chemical tags facilitate the simultaneous analysis of 2 to 11 experimental samples. Applications Isobaric labeling has been successfully used for many biological applications including protein identification and quantification, protein expression profiling of normal vs abnormal states, quantitative analysis of proteins for which no antibodies are available and identification and quantification of post translationally modified proteins. Availability There are two types of isobaric tags commercially available: tandem mass tags (TMT) and isobaric tags for relative and absolute quantitation (iTRAQ). Amine-reactive TMT are available in duplex, 6-plex and 10-plex and now 11-plex sets. Amine-reactive iTRAQ are available in 4-plex and 8-plex forms. References Mass spectrometry